On 02/18/2011 09:35 AM, xorbe wrote:
#include<stdio.h>
char b[4] = {0,0,0,0};
const short*w((short*)b);
const int*d(( int*)b);
void print() {printf("%02x%02x%02x%02x %04x%04x %08x\n",
b[3],b[2],b[1],b[0],w[1],w[0],d[0]);}
int main() {
print();
for (int i(0); i<4; ++i) { b[i] = i+1; print(); }
}
You're right. This might make it clearer, it's a version that's valid C
and looks like it has an optimization opportunity (which would give you
output you _didn't_ want;) but actually doesn't because of the special
case char& or char* can alias anything rule.
#include <stdio.h>
char b[4] = {0,0,0,0};
const short *w;
const int *d;
int main()
{
size_t i;
w = ((short*)b);
d = (( int*)b);
/* the next line makes explicit endianess assumptions */
printf("%02x%02x%02x%02x %04x%04x
%08x\n",b[3],b[2],b[1],b[0],w[1],w[0],d[0]);
for (i=0; i<4; ++i) {
b[i] = (char)(i+1);
printf("%02x%02x%02x%02x %04x%04x
%08x\n",b[3],b[2],b[1],b[0],w[1],w[0],d[0]);
}
return 0;
}
A compiler might want to optimize the references to w and d since
they're unchanged inside the loop, and just remember their values from
outside the list. Unfortunately, since access through a char reference
or pointer can alias anything, the compiler is unable to make this
assumption and cannot do any optimizations. In general, doing any
accesses of things through a char* or char& in a method or function can
make a lot of optimizations impossible.
Patrick
p.s. Please read my aliasing whitepaper and make any suggestions for
improvement that occur to you. http://dbp-consulting.com/StrictAliasing.pdf
